1. Field of the Invention
The present invention relates to a method of processing signals, and is particularly concerned with a method of processing signals for use in the detection, location and tracking of objects through the processing of the output signal from an object detection system.
2. Related art
Numerous sensor systems have been proposed for detecting the presence of objects and determining whether their position or trajectory represents a hazard either to themselves or to other objects nearby. Examples of such proposals are to be found in the fields of automotive collision warning, robotic guidance, missile test and defence, domestic intrusion and safety alarms.
The ultimate purpose of such prior art systems is normally to give an indication that some form of action needs to be taken as a result of the measured position or trajectory of a single detected object.
Prior art sensor systems generally consist of a detector of some form whose output signals are provided as an input to a signal processor. The detector is capable of detecting aspects of objects relating to their position, size, speed etc. and uses an appropriate transducer to generate an output signal which represents a characteristic or the behaviour of a detected object. Appropriate transducers include, for example, radar transceivers, lidar transceivers, ultrasonic transceivers, infrared transceivers or cameras or video cameras.
FIG. 1 shows a typical sensor output signal at a particular moment in time. Detection is achieved by the setting of a threshold, as indicated in FIG. 1. An object is considered to have been detected if the output signal which it causes from the sensor system is above a certain predetermined level, i.e. the threshold.
In addition to merely detecting presence of an object, some prior art systems are capable of some form of discrimination as to which detected objects are xe2x80x98of interestxe2x80x99 in a particular implementation and which objects are not. Objects of interest may be differentiated from objects which are not of interest by monitoring and processing of any one of several measurements associated with the object (or a combination thereof). Appropriate measurements for such differentiation include, for example, a sensor signal strength, a measured distance, a measured direction, and a measured or calculated speed of motion.
A major problem in prior art sensor systems is coping with fluctuations in the output signal of the transducer caused by phenomena other than the movement of the detected object. In practice, the signal which represents the presence of the object may fluctuate in amplitude, in frequency, in delay or phase, and in particular may exhibit glint or fading when suitable propagation conditions exist. Prior art systems generally take momentary measurements of a single target object.
These fluctuations lead to problems with prior art systems sometimes recognising objects which are not really xe2x80x98of interestxe2x80x99 to the system as if they were of interest. Even more critically, the fluctuations may sometimes lead to a prior art sensor system losing track of an object and/or not detecting a particular object at all.
Some prior art systems overcome some of the problems with fluctuation by altering the level of the threshold in order to track a detected object. That is to say, once an object has been recognised as having been detected within the field of view of the sensor system, the system automatically ensures that the detected object is tracked as long as it remains in the field of view of the system. In order to achieve such tracking, these prior art systems react to a loss of object detection (e.g. caused by fluctuation of a signal) by lowering the threshold level until a signal is detected and react to a signal which is fluctuating to a large amplitude by raising the threshold level.
Such threshold raising/lowering works reasonably well for situations in which there is one clear object which is to be detected. However, when there is more than one object in the field of view, such threshold raising/lowering causes problems. For example, in the situation shown in FIG. 2, if the system raises the threshold to take account of the signal level associated with the first object, then the system will lose track of the second object.
It is an object of the present invention to provide an improved method of processing signals from an object detection sensor.
It is an advantage of at least some embodiments of the invention to be able provide a method which is capable of reliably establishing continuity of position or trajectory for multiple objects in a field of view.
It is a further advantage of at least some embodiments of the present invention to be able to provide a method of processing signals from an object detection sensor in which such signals associated with one or more objects may be processed to provide a robust estimate of position or trajectory, the estimate being relatively insensitive to fluctuations in the measured signal, and the estimate of trajectory or track in time, allowing the behaviour of the object or objects to be described as a function of time, this allowing a prediction of position or hazard to be carried out, or further analysis to be made to aid in classifying the object.
The present invention provides a method of processing a signal, such as an output signal of an object detection sensor, which signal comprises a plurality of amplitude values each associated with a respective value along a reference axis which is representative of a physical quantity, such as the distance of an object from an object detection sensor, the method comprising the steps of:
providing a detection-threshold function comprising a plurality of detection-threshold values each associated with a respective value along said reference axis;
comparing the signal to the detection-threshold function;
identifying target regions of the reference axis where the signal exceeds the detection-threshold function;
in response to the identification of a target region, reducing the detection-threshold values of the detection-threshold function within said target region.
In further aspects the invention also provides a radar system which uses this method and a land-based vehicle, an air-bag deployment system and a control system which uses such a radar system.
In a further aspect, the present invention also provides computer software which, when run, carries out this method.
The method of the present invention elegantly overcomes the problems of prior art methods.
The method of the present invention allows the reliable tracking of a plurality of objects in a field of view.
Unlike prior art methods, the method of the present invention adapts its threshold values separately for each detected object and thus ensures that objects are neither missed nor misrecognised.